Special Report

Design and of Linn Cove

Jean M. Muller Chairman of the Board and Technical Director Figg and Muller , Inc. Tallahassee. Florida

James M. Barker Assistant Vice President/Projects Figg and Muller Engiineers, Inc. Mt. Pleasant, South Carolina

he Blue Ridge , owned by so visitors will have an unobstructed Tthe United States National Park Ser- view as they journey through the 479 vices (NPS), is a unique transportation/ mile (755 km) park. recreation network. Built along the crest Construction of the Parkway began in of the beautiful Blue Ridge 1933 and was completed in the late in , the 469 mile (755 km) 1950's with one exception: a 5-mile (8 long attracts millions of vaca- kin) length around privately owned tioners each year with its beautiful Grandfather , which is one of scenery and facilities for camping, hik- the leading tourist attractions in North ing and picnicking. Carolina, The had Through the years the NPS has made settled numerous location and right- every effort to preserve the natural of-way problems through the years, but beauty of the roadway and to provide ac- none approached the magnitude of that cess to adjacent areas with parking areas centered on , and nature trails. All highway construc- The owner of Grandfather Mountain tion is in accordance with the "park con- was insistent that construction of the cept." For instance, fill must be placed Parkway must not harm or be obtrusive and granite clad retaining walls built to the natural scenic beauty found there. before the side of a mountain can be cut. Since the owner shared the same All are built with stone-clad philosophy, the Federal Highway Ad- and wingwalls to blend with the ministration (FHWA), which supplies surrounding mountain landscape. services for the Parkway, Bridges must also have open handrails studied alignments and alternate routes

38 Several innovative design and construction techniques were used to build the Linn Cove Viaduct — a 1243 ft (379 m) long, S-shaped precast prestressed segmental — situated in one of the most scenically beautiful regions of the United States.

PCI JOURNALISeptember-October 1985 39 Fig. 1. Finished view of the Linn Cove Viaduct set against the beautifully rugged . for approximately 10 years. the site, it generally occurred below Finally, a route which could not be a relatively shallow overburden of seen from the mountain top was chosen soil surfaced with decomposed vegeta- around the eastern mountainside. To tion. minimize construction damage to the The proposed for this project area, the roadway alignment was raised was a precast post-tensioned segmental above the mountainside by use of a via- concrete box superstructure, eight spans duct structure. in length with both horizontal and verti- Since building the viaduct structure cal curvature conforming to the natural by conventional methods of bridge con- topography. Progressing from south to struction would have done irreparable north, the spans are 98.5, 163 ft (30, 48 damage to the environmentally sensi- m), four at 180, 163 and 98.5 ft (55, 50 tive area, the search for an acceptable and 30 m) in length. Each span is com- construction method became a chal- prised of 9 ft (2.7 rn) deep single cell lenging engineering problem. boxes of 8.5 ft (2.6 m) nominal length located between 5 ft (1.5 m) long DESIGN segments. The Linn Cove Viaduct is probably The site within the limits of the Linn one of the most complex bridges ever Cove Viaduct (Fig. 1) is a rugged and built; certainly, it must be the most steeply sloped terrain with relatively complicated concrete heavy ground vegetation and boulder ever constructed. Three major factors out-croppings. A natural stream crosses contribute to its complexity: environ- the bridge near the northern end. The mental constraints and inaccessibility of scenic location, with its protruding the site and the vertical and horizontal boulders and the requirement for their alignment. protection and preservation, was the The bridge was literally built on the major factor to be considered in the de- side of a mountain which had to remain sign. Although rock existed throughout in its natural state. There was only one

40 Fig. 2. Construction overview of Linn Cove Viaduct. This bridge is the first segmental structure in the United States to incorporate progressive placing erection.

way in or out, namely, over the com- mations. pleted portion of the bridge. The hori- The bridge is 1243 ft (379 m) in over- zontal alignment includes spiral curves all length with a -to-curb roadway going into circular curves with radii as width of 35 ft (11 m). The final roadway small as 250 ft (76 m) and with curvature surface is a 2 in. (51 mm) bituminous in two directions, which gives the overlay with waterproofing membrane. bridge its S shape (Fig. 2). A small por- The structure was designed in 1979 in tion of the bridge is on a horizontal tan- accordance with the AASHTO Standard gent. The superelevation goes from a Specifications for Highway Bridges full 10 percent in one direction to a full (Twelth Edition 1977) except for those 10 percent in the other direction in 180 design and construction techniques ft (55 m) transitions and part way back particular to post-tensioned segmental again within the length of the bridge. construction. All superstructure and No two of the 153 superstructure seg- substructure precast segments were de- ments have the same dimensions, and signed for a 28-day concrete strength of only one of the segments in the entire 6000 psi (41 MPa) with 4000 psi (28 bridge is straight. When the vertical MPa) specified for release of the forms. curve and tangential alignment are con- In addition to the longitudinal post- sidered, the Linn Cove Viaduct includes tensioning tendons, the top slab was de- almost every kind of alignment signed to be transversely post- geometry used in highway construction. tensioned, All permanent post- The foundations consist of cast-in- tensioning was done with' in. (13 mm) place abutments at each end and seven diameter, 270 ksi (1863 MPa), low re- intermediate precast segmental box laxation strand tendons. piers bearing on 20 ft (6.1 m) diameter To minimize the environmental im- footings. Both the abutments and pier pact on the construction site, the major footings are supported on reinforced 9 portion of the bridge was to be built with- in. (229 mm) diameter microshafts out the use of access . The only drilled into the underlying rock for- permitted construction was from

PCI JOURNAL September-October 1985 41 the south abutment of the second pier, a No trees other than ones directly be- distance of approximately 260 ft (79 m). neath the bridge were allowed to be cut. From this pier to the end of the bridge, Each tree had to be evaluated separately the construction was to be from the pre and approved for cutting. All foliage ad- -viously completed portion of the . jacent to the bridge had to be protected

li TT rTTT'T'T -FT-F IL LLILLLi LJ_LJ Permanent __.4.f[ Permanent Pier Support ^ Pier J,LL

Fig. 3. Schematic view of the progressive placing erection method.

HALF SECTION AT TYPICAL HALF SECTION POST-TENSIONING BLOCK THRU SEGMENT

Fig. 4. Typical cross section of precast segment. The webs and bottom stab were thickened to carry the stresses due to the extreme curvature. Both temporary and permanent post-tensioning tendons were anchored at the intermediate stiffeners shown on the left side of the drawing.

42 by a silt fence located along the entire beautiful site of Linn Cove, it was de- length of the bridge. Any construction cided to use a span-to-depth ratio of 22 debris on the outside of the silt fence to provide a graceful structure. How- had to he immediately retrieved. ever, after analysis of this cross section, None of the boulders could be de- it was decided to increase the depth of faced during construction, except in in- the box and the web thicknesses be- stances of rock bolting. The boulders cause the shallower depth structure were covered to prevent concrete, grout would have required a significant or epoxy stains. Any extraneous material amount of high strength post-tensioning on the boulders was immediately steel, Fig. 4 shows the final cross section cleaned off. adopted. The streams flowing across the bridge construction with large alignment were protected from siltation equipment weights applied at the free or other contamination. Water quality end induces high compressive stresses was constantly monitored. in the bottom slab. This in combination Because of the severe environmental with shear stresses resulting from the restrictions, the bridge could not he torsional moment and the longitudinal constructed by conventional methods shear force, results in a bottom slab and a system was devised in the design thicker than the usual 12 in. (305 mm) to construct it from the top. This re- for the typical section and a web width quired a progressive scheme which en- of 18 in. (457 mm). abled segments of the bridge to he Through the use of Figg and Muller's transported across the previously built exclusive Bridge Construction (BC) deck and assembled into final position. Program, it was possible to analyze the Precast concrete was chosen over structure with a discrete model corre- cast-in-place segments because the re- sponding to the joints between seg- gion has a relatively short construction ments (153 segments). Each stage of season. By choosing precast concrete, erection was analyzed. Stresses were production of the segments could con- checked at each segment placing, ten- tinue during the winter. Additionally, don stressing or support condition the precast segments were made under change. In addition, certain critical eon- plant-controlled conditions which led to struction phases were analyzed, for in- high quality concrete. stance, loads induced by trucks carrying The progressive scheme (shown in segments over the completed deck. Fig. 3) is considered feasible and struc- The primary advantage of the BC pro- turally satisfactory for span ranges of gram was its ability to handle long-term from 150 to 200 ft (46 to 61 m). For these behavior computations. The time- span lengths, a constant depth box gir- dependent deformations were antici- der has proven to be the most economi- pated to he important because of creep, cal. After visits to the site and analysis of shrinkage and steel relaxation. the possible locations of piers and tem- The long-term deformations calcu- porary supports, the maximum span lated by the BC program have been length was established at 180 ft (55 m). found very consistent with actual mea- Spans were determined after locating surements of deflections in the past. The piers to avoid the natural out-croppings accuracy was again verified with field of rock. measurements on Linn Cove. The pro- Typical box girder bridges have been gram uses the FIP-CEB concrete creep built with span-to-depth ratios of 18 to provisions. 25. Structural aesthetics are generally The transverse analysis was per- more pleasing through the use of higher formed by utilizing the STRUDL pro- span-to-depth ratios. Considering the gram to analyze a three-dimensional fi-

PCI JOURNALJSeptember-October 1985 43 Core Form j Steel Bulkhead To Storage

New Segmen t Old Older (N) (N-I) (N-2) Soffit Carriage

Fig. 5_ Schematic of short line match-casting system as used on Linn Cove Viaduct. nite element model. It should be recalled all critical phases, i.e., completed can- that Linn Cove was designed in 1978 tilever with equipment loadings or deck and 1979. Figg and Muller has de- behavior at temporary support removal. veloped and now uses the much more powerful HERCULE program for CONSTRUCTION three-dimensional finite element analysis. Prior to discussing the precasting op- The effects of the bridge horizontal eration and erection sequence, some curvature were analyzed using two ad- mention needs to be made about the ditional three-dimensional computer contract administration of the project. programs: • A curved beam program (ABC) to Contract Administration analyze the general behavior of the structure during construction and at ser- The National Park Service (NPS) is vice. This program allows the computa- the owner of the project and assumed tion of load distribution in accordance overall responsibility. The NPS inspec- with all element stiffness (box girder, tion responsibility related primarily to bearings, piers and temporary supports). control of environmental aspects. In- • A finite element analysis program spection was accomplished by periodic (TITUS) was used for the detailed de- visits to the construction site. NPS per- sign of torsional stresses in all members sonnel were also present at all con- of the girder. In particular, this program struction meetings. was used to design the diaphragms (at The Federal Highway Administration piers and temporary supports) which (FHWA) assumed responsibility for transfer the torsional load to the bear- contract administration. A full-time staff ings. on the construction site perfomed all in- Torsional analysis was performed for spection work involved with microshaft

44 pile- and footing construction, shown in Fig. 6. This method involved Superstructure and pier stein casting casting the new segment above the pre- and erection were inspected with the viously cast segment with steel forms assistance of Figg and Muller En- used for the side forms and a core form gineers, Inc. The contractor for the proj- to cast the void in the box. The rein- ect was Jasper Construction Company of forcing cages (including the vertical , Minnesota. The contrac- tendon ducts) were prefabricated. tor's successful bid on the project was The only geometry involved in cast- $7.9 million. ing the pier segments was the determi- Figg and Muller Engineers de- nation of the as-cast data and making veloped the original concept of the con- minor corrections for casting variances. struction method. The y also did the final The as-cast data were later used when design and prepared all contract docu- erecting the box pier segments. ments. Because of the complexity of the The severe geometry requirements of project and relatively short time since Linn Cove necessitated some unique the introduction of the segmental con- properties for the superstructure seg- cept in the United States, the FHWA ment casting . The system had engaged Figg and Muller Engineers to to be strong enough to support 55 tons provide the necessary segmental tech- (50 t) of concrete and steel; yet flexible nical expertise to assure successful enough to seal around the edges of the completion of the project. cast-against segment, which in some in- The engineers served in strictly an advisory capacity to the FHWA, with no direct relation to either NPS or the con- tractor. All functions related to the specialized techniques required for segment casting and erection. 1 .r• L Segment Casting To tit curvature and superelevation variation requirements, segments were laid out for the short cell method with match-cast joints. Each segment was cast in casting between a bulk- head and the previously cast segment. Fig. 5 shows the short cell match-cast arrangement where N-1 represents the match-cast segment and N, the segment being cast. Horizontal curves, vertical curves and superelevations are obtained by adjust- ing the position of the match-cast seg- ment. This procedure requires a special casting form designed to allow defor- mations between the bulkhead and the match-cast segment. Curves are accom- plished by successive chords providing a slight angle change at each joint. The precast pier segments were match-cast in the vertical direction as

PCI JOURNALJSeptember-October 1985 45 stances was severely skewed because of Casting Geometry Control the large degree of curvature to be ob- tained. The contractor had ultimate responsi- To assist in obtaining adequate seal- bility for the geometry control of the ing to ensure smooth joints, the con- segments during the casting operation tractor installed a strip of rubberized and during erection of the . material on the edge of the casting The contractor developed theoretical machine. This worked very well with casting curves from the contract docu- the only detrimental effects being occa- ments and submitted the data to the sional pieces of rubber which would FHWA, which consulted Figg and stick to the concrete. However, the Muller Engineers before giving ap- pieces were easily removed. The casting proval. machine is shown in Fig. 7. The engineers established a com- The extreme weather conditions on pletely independent geometry control Grandfather Mountain of below zero system from that used by the contractor. temperatures and winds ranging to 100 The primary function of this separate miles per hr (161 krn/hr) influenced the evaluation was to provide a system of contractor to enclose the casting checks and balances, thereby assuring machine and reinforcement cage fabri- complete and accurate geometry con- cation area in a building. However, since trol. the box pier segments were relatively The three types of movement that can few in number, they were cast outside. be required of a casting machine were

Fig. 7. The casting machine was located in a building enabling segments to be cast during the winter months. The next cast-against segment can be seen in the foreground; and, behind that, the steel bulkhead can be seen. The workers are standing on the core form which moves in to form the box girder void.

46 all required on this project, and to a much greater degree than on any previ- ous bridge. They were; • Movement in plane to obtain a curved alignment. • Movement in elevation to obtain the desired vertical profile. • A warping movement in the form soffit to obtain the desired cross fall variations. The theoretical casting curves are those curves to which the segments are assembled in the bridge so that they will conform to the geometric shape shown on the contract drawings. The theoreti- cal casting curves, therefore, have to take into account the geometric align- ment and vertical profiles and the ef- fect of the deflections in the superstruc- ture that will occur during and after erection. For a straight bridge, the casting curve is simply the same information on the contract drawings. However, in the case of Linn Cove, whenever any segment cast basically horizontal in the casting cell had to be rotated about its longi- tudinal axis to be placed in the bridge, the geometric profile of the segment and all its adjacent segments were very different to the alignment and pro- file shown on the contract draw- ings. Fig. 8. The contractor chose to erect the segments with this stiffleg attached Normal geometry control procedures to the end of the cantilever. This single could not be used. The geometry of piece of equipment controlled the Linn Cove with 21 segments between construction rate of the bridge. All material cast-in-place closure joints would have and equipment was handled with this resulted in the twenty-first segment crane. being approximately 4 in. (102 mm) off line and 6 ft (1.8 m) off grade, had not the bridge and casting cell angular change differential been taken into ac- Segmental Box Pier count Construction For bridges of larger radii and 3 per- cent superelevation, the effect is insig- All pier footings are 20 ft (6.1 m) in nificant in alignment, but not profile. diameter and 5 ft (1.5 m) in thickness. For this bridge, it was necessary to gen- The footings are founded on variable erate "geometry" casting curves from thickness nonreinforced subfootings "global" data on the contract drawings through which microshaft piles were by transformation to the local "casting drilled. cell" axis. After forming the footings and placing

PCI JOURNAUSeptember•October 1985 47 four points. The contractor was responsible for aligning the segment and the alignment was subsequently verified by the en- gineers. After verification, the footing was cast. The top of the footing ex- tended approximately 1 in. (25 mm) above the bottom of the segment and the support beams for the segment were lost. After the footing concrete had hard- ened, the joint between the cast-in- place concrete and the precast segment was pressure grouted with epoxy. The purpose of the epoxy was to waterproof the joint. The epoxy was not related to strength. The precast box pier seg- ments were delivered over the com- pleted portion of the superstructure which extended to within two segments of the pier location. The segments were lifted over the end of the cantilever by a stiffleg crane attached to the cantilever (see Fig. 8). Fig. 9 shows a box pier segment being lowered. The pier segments were blocked about 6 in. (152 mm) above the previous segment while epoxy was applied. The segment was then lowered Fig. 9. Precast box pier segments were to the face of its match-cast unit where lowered over the end of the completed thread bar tendons were installed and cantilever with the stiff leg. The workmen stressed. Each pair of segments were are ready to block the segment and apply stressed together with thread bars. The the epoxy. process was repeated until all of the hollow segments were erected and the pier was ready for the cap. the reinforcement and post-tensioning The last step in the pier construction conduits, the first of the precast seg- was to place the cap and stress eight 12- ments was placed. A steel frame fabri- strand tendons. These tendons extend cated from rolled sections was placed in from the top of the pier down through the footing simultaneously with the and out the side of the footings. Once reinforcing bars. The frame provided stressing was completed, the tendons support for the first box pier segments. were grouted. The initial precast segment was placed on shims and jacks which enabled Superstructure Segment proper alignment. The segment had to Erection he aligned to correct for casting varia- tions and actual superstructure position. Superstructure segments were deliv- Movements were achieved by pumping ered to the end of the cantilever by a the flat jacks one at a time or in combi- low-boy tractor trailer (see Fig. 10). Be- nation. The segment was supported at cause of the limited access, the truck

48 had to be backed up the access road anti backed out onto the completed portion of the bridge. The crane which lifted the segments from the truck and swung them around to the end of the cantilever was an American S-20 stiffleg crane. It was equipped with a 70 ft (21 m) boom and provided a 125 kip (556 kn) lifting capacity at a 25 ft (7.6 m) boom radius. Fig. 10. 50 ton (45 t) segments had to be The crane had to be moved forward backed for distances exceeding 1 mile (1.6 after each segment was erected. Gener- km) in many instances because there was ally, it was located two segments behind insufficient room to turn this rig around. the segment being erected. The moving operation involved lifting the entire as- sembly, removing the steel support beams and lowering the assembly onto steel rollers. Once the crane was sup- ported on rollers, it was pulled forward by hand-operated winches. Then the lifting process was reversed. The steel support beams were reinstalled and the crane was tied down. The moving operation cycle took 4 to 6 hours to complete. Moving the stiffleg crane entirely controlled the erection rate of superstructure segments. Incor- poration of a swivel crane as detailed on the contract drawings or modifying the stiffleg crane for faster moving would have substantially increased the super- structure erection rate. The following is a step-by-step proce- dure followed during the erection of a typical segment: Step I — The segment was backed to the end of the bridge by truck (see Fig. Fig. 11. The first step in the erection 10). sequence was to remove the Step 2 — The stressing cages were post-tensioning stressing cage from the picked up by the stiffleg and attached to end of the cantilever. The permanent the new segment (see Fig. I U. tendon live end anchors can be seen at the Step 3—The lifting spreader beam top of the webs. was lifted with the stiffleg and the lifting cables attached to the new segment. Step 4 — The segment was lifted to clear the trailer and the slope was to within about 6 in. (152 mm) of the end checked to see that it matched the de- ofthe cantilever (see Figs. 12, 13, 14 and gree of superelevation at the end of the 15). cantilever. If necessary, it was adjusted. Step 6— The segment was pulled Step 5 — The segment was lifted by horizontally to the end of the cantilever the stiffleg, swung around and lowered with cable winches and blocked 6 in.

PCI JOURNAUSeptember-October 1985 49 Fig. 12. The segment was lifted from the truck on the right and swung out over the cantilever end.

Fig. 13. Then the segment was lowered vertically.

50 Fig. 14. Here the segment is in place ready to receive the epoxy bonding agent. The excellent fit of the previous joints and the curvature can be seen. Most joints were less than 1 mm (0.004 in.) in thickness.

(152 mm) away with wood blocks. It was still supported vertically by the stiffleg, Step 7 — The temporary thread bar tendons were threaded and the nuts finger tightened. Step B — The epoxy was mixed and applied. Step 9 — The wooden blocks were removed and the segment closed to the end of the cantilevers with the cable winches. Step 110 — The temporary thread bars were stressed. Step 11 — The permanent 19-strand tendons were threaded and stressed. Step 12 — The temporary thread bar tendons were released. Because erection of the superstruc- ture continued through the winter, pro- visions were taken to heat and insulate the joints of the precast segments. In particular, measures were taken to be sure the epoxy in the joints was heated Fig. 15. Erection progress (May 20, 1982).

PCI JOUPNAL/Septernber-October 1985 51 to at least 40°F (4.4°C) in order for the material to cure properly. Prior to con- struction, an elaborate testing program was undertaken to make sure that the heating/insulating system worked prop- erly. For more details, see the article listed below.*

Erection Geometry Control Figg and Muller's responsibility for erection geometry control was to advise the FHWA of the bridge's relative posi- tion to the plan and profile grade lines. The responsibility included verification of the positioning of bent segments by the contractor prior to casting closure joints and the positioning of the bottom pier shaft segments. During erection, the bridge was tracked using the survey control points Fig. 16. Linn Cove Viaduct winding through cast into the top slab of the segments. the Blue Ridge Mountains. The expected erection grades were pre- determined at each stage of construction for the three leading joints of the canti- lever. These grades consisted of a sum- mation of the desired final grade, the as-cast variances and the cantilever camber data. The actual elevations of the joints were plotted on graphs for comparison with expected elevations. It was determined early in the erec- tion that the bridge was moving too much to acccomplish accurate horizon- tal tracking with the instrument on the bridge. The theoretical horizontal posi- tion of the cantilever end was deter- mined by calculating coordinates from the base grid on the plans. The actual coordinates of the cantilever end were determined by turning angles and measuring distances from known points. The specified erection tolerance was ±1 in. (25 mm) in both the vertical and horizontal directions. An early decision was made that with the many adjust-

'Muller, lean, and Barker, James M., "Joint Heat- ing Allows Winter Construction on Linn Cove Fig. 17. Closeup of one of the many turns Viaduct," PCI JOURNAL, V. 27, No. 5, Scptcm- of the bridge. ber-nctnher 1982, pp. 120-130.

52 ments available at closure joints and CONCLUDING aligning box pier segments, there would be no need to shim the segments. REMARKS Shimming was only to be used when no The Linn Cove Viaduct is a vital part other way out could be foreseen, and of the missing link of the Blue Ridge with this type of erection there was al- Parkway. Connecting roadways will ways another way out. Shims were complete that link by 1987, allowing the never used. general public access to an award win- The 250 ft (76.2 m) radius curves, spi- ning example of engineering ingenuity: ral curves, and many superelevation A truly innovative bridge by design, transitions were negotiated without a built under difficult conditions, and set significant problem. The rigid controls. in complete harmony with its natural exercised during casting made the erec- surroundings. Attesting to its success, tion process very smooth. the project has won eight national de- The last segment was placed on De- sign awards including a PCI Awards cember 22, 1982. During 1983, the con- Program winner in 1983. taractor cast the curbs, completed the last abutment, finished the grade on the north end of the viaduct (inaccessible CREDITS before the bridge was erected), installed guardrails and applied the waterproof- : Figg and Muller Engineers, ing membrane and wearing surface. Inc., Tallahassee, Florida. The bridge was completed without a Foundation Design/Construction Ad- single structural problem. There is not a ministration: Federal Highway Ad- crack in any of the precast superstruc- ministration, Eastern Division, Direct ture or substructure segments. The proj- Federal Projects, Arlington, Virginia. ect is a tribute to what can be accom- General Contractor: Jasper Construc- plished with proper segmental design tion, Inc., Plymouth, Minnesota. and construction techniques, Figs. 16 Owner: National Park Service, Denver, and 17 show the completed bridge. Colorado.

PCI JOUR€NALJSeptember-October 1985 53